# Cosmology, Decoherence and the Second Law

**Authors:** Sebastian Cespedes, Senarath de Alwis, Fernando Quevedo  
**arXiv:** [2509.07077](https://arxiv.org/abs/2509.07077)  
**Date:** 2025-09-08  
**Summary page:** https://www.summarizepaper.com/en/paper/2509.07077v1/

## Key points

- Study titled "Cosmology, Decoherence and the Second Law" by Cespedes, de Alwis, and Quevedo
- Investigate quantum decoherence and entropy increase in early universe cosmology
- Decoherence in discrete bipartite quantum system leads to entropy increase
- Comparison with continuous systems relevant to cosmology, including inflationary fluctuations
- Off-diagonal terms of density matrix may not decay in field bases
- Entanglement entropy for Gaussian states shows monotonically increasing trend during accelerated phases
- Extension to non-Gaussian states reveals dynamics of entropy growth
- Emphasis on computing von Neumann entropy over thermodynamic entropy and analyzing contributions from CMB modes
- Future work involves comprehensive examination of various types of entropies and unitary evolutions governed by Liouville's theorem

## Summary

In their study titled "Cosmology, Decoherence and the Second Law," Sebastian Cespedes, Senarath de Alwis, and Fernando Quevedo delve into the intricate relationship between quantum decoherence and entropy increase in early universe cosmology. The researchers first investigate decoherence in a discrete bipartite quantum system where a single qubit becomes entangled with an environment. This leads to an increase in entropy that is linked to the decay of off-diagonal terms in the reduced density matrix. Drawing comparisons with continuous systems relevant to cosmology, particularly those involving sub- and super-horizon inflationary fluctuations, the team explores how external interventions impact the evolution of entropy. Contrary to assumptions in some cosmological setups, they find that the off-diagonal terms of the density matrix do not necessarily decay in field bases. By employing open Effective Field Theories (EFTs), Cespedes et al. compute the entanglement entropy for Gaussian states and demonstrate a monotonically increasing trend ($\dot S>0$) during accelerated phases ($\ddot a>0$, with $a(t)$ representing the scale factor). This result is extended to encompass non-Gaussian states, shedding light on the dynamics of entropy growth within such systems. Moreover, the researchers touch upon the significance of computing von Neumann entropy as opposed to thermodynamic entropy and emphasize that their focus primarily lies on analyzing contributions from CMB modes. They hint at future work involving a more comprehensive examination encompassing various types of entropies and unitary evolutions governed by Liouville's theorem. In conclusion, while acknowledging that fundamental questions regarding entropy in our universe remain unresolved, Cespedes, de Alwis, and Quevedo provide valuable insights into specific aspects by delving into entropy arising from inflationary CMB modes. Their exploration sheds light on natural separations between system and environment within cosmological frameworks and highlights potential avenues for further research bridging different approaches within this complex domain.